I don't know what exactly are Bronze, Silver, Gold and Platinum elos. Does it have any other meaning than just a way to tell people how good you are?
WARNING: This answer was based on a "leaked" list located on the forum which eventually turned out to be a hoax. The newly released information is available on the LOL forums The actual ELO tier system has changed significantly for season 2.
Along with the bragging rights, at the end of a season rewards are given to each elo. The season 1 rewards were the following:
Platinum (Top 0.2%) - 1900 and above (3v3: 1700+, pre-made 5v5: 1750+): a framed summoner icon in platinum, a platinum banner in summoner profile, a platinum forum badge and a special skin for Jarvan IV
Gold (Top 3%) - Between 1520 and 1899 (3v3: 1490-1699, pre-made 5v5: 1500-1749) : a framed summoner icon in gold, a gold banner in your summoner profile, a gold forum badge and a special skin for Jarvan IV
Silver (Top 10%) - Between 1400 and 1519 (3v3: 1410-1489, pre-made 5v5: 1410-1499) : a framed summoner icon in silver and a silver banner in your summoner profile
Bronze (Top 25%) - Between 1249 and 1399 (3v3: 1249-1409, pre-made 5v5: 1249-1409) : a bronze banner in your summoner profile
This information was a hoax perpetuated on the forums, and is confirmed false:
The leaked season two list is as follows, but its not confirmed to be real:
Bronze [1250-1399] 25% - Glorious Poppy [Skin] and a second layer of bronze frame on profile.
Silver [1400-1519] 10% - Regifted Wukong [Skin] and a second layer of silver frame on profile.
Gold [1520 - 1899] 2% - Vandal Sivir [Skin] and a second layer of gold frame on profile.
Platinum [1900+] 0.2% - Diamond Anivia [Skin] and a second layer of platinum frame on profile.
Each layer gets the skins from preceding layers as well. If a player does not have the champion for a skin, he also receives the champion.
Also, there is a fifth layer called "True platinum" at 2220+, where platinum originally started. It is visible as the platinum badge glows brightly on the profile screen. True plat has no rewards, it was added to keep 2220+ players happy when platinum was made easier to acquire. Season 1 true plat players received recognition on the website.